This invention relates to running braces and in particular to energy efficient running braces.
This invention augments the effective spring constant of the leg by adding a resilient brace which supports the runner's weight in parallel with the leg.
The act of running involves vertical motion of a runner's center of gravity. The lifting of the runner's weight requires muscle work. When the runner's foot impacts the ground, both the kinetic energy and the momentum associated with the vertical motion must be absorbed. Approximately 45% of the vertical kinetic energy is stored in the resilient parts of the leg and foot, but most of it is lost to the ground and the leg. The lost energy must be replaced by muscle work. This invention is intended to minimize lost energy or, equivalently, to maximize the energy efficiency of running.
Scientific inquiry of running includes the study of the efficiency of running as a function of various parameters. Researchers have been attempting to discover optimized running parameters for greater energy efficiency and fewer injuries. Dr. Thomas A. McMahon of Harvard University discussed how the resiliency of tracks can be tuned to improve performance and safety in his article "Mechanics of Locomotion," T. McMahon, 3 Int. J. Of Robotics Research 4 (1984). One of his conclusions is that running times improve by two or three percent and injuries are reduced by a factor of two when the effective spring constant of the track is approximately two times that of a runner's leg. Prior art running shoes cannot achieve improved energy efficiency equivalent to that achieved with tuned tracks, because impact is on the heel whereas take-off is from the toe. Prior art running shoes do not have a means for transmitting the impact energy from the heel to the toe.
Furthermore, the effective spring constant of a leg must be large to achieve high performance, or speed. A drawback of tuned tracks is that the effective stiffness of the leg/track system is smaller than that of the leg itself, since the track acts as a spring in series with the spring representing the leg, and the spring constants of springs acting in series add reciprocally. The present invention solves that problem by having the braces act in parallel with the legs, in which case the spring constants add linearly.
Two important concepts are compliance and resilience. Compliance refers to the property of the sole to give or compress upon foot impact; resilience refers to the property of the sole to return to its original shape. This can be made clearer by referring to a spring model with damping. The term damping includes all friction losses. A spring system may be very compliant by virtue of having considerable damping, but then it is not energy efficient. Prior art running shoes have this drawback. The term resilience as used herein means that damping is minimized, so energy efficiency is maximized. In summary, compliance describes a system where impact energy is dissipated, whereas resilience refers to a system where energy loss is conserved.
Leg-brace devices in the prior art are called walking irons. U.S. Pat. No. 2,206,234 discloses a brace which extends from the foot to the upper leg and it has a spring to cushion the impact of foot strike. However, contact with the ground is made via a pair of feet, each of which is similar to the foot on a pogo stick. This prior art is appropriate for hobbling, whereas the present invention is intended for running--in that the foot strike involves the entire foot, thereby making possible better balance and stability. Another difference is that the cushion springs in the just-mentioned invention act in series with the effective springs of the legs, whereas the brace springs in this invention act in parallel, which leads to higher values of effective spring constant, and which makes possible greatly enhanced performance.
Around 1890, five running brace patents were granted to Nicholas Yagn, a mechanical engineer in the army of the Emperor of Russia, with U.S. Pat. Nos. 420,178; 420,179; 438,830; and 440,684. The first two of these use bow springs, attached to the shoulders and to the pelvis, respectively, to provide parallel support to the legs in running, and, as such, are distinct from the present invention. The third does not provide parallel support to the legs, and, hence, is distinct from my invention. The fourth is based on the unworkable concept that a flexible tube could be made to function like a bow spring by filling it with a sufficiently high-pressure gas.
The fifth patent, U.S. Pat. No. 406,328 describes a brace which provides parallel support for the leg while running. The brace consists of two telescopic members which act against a spring to store the energy associated with the runner's vertical motion. It also provides means for the runner to bend one knee slightly, without compressing the storage spring, during recovery, while the other leg is in contact with the ground. These provisions are necessary, but not sufficient, components of a viable running brace for the following reasons.
The first major drawback in the telescopic design of U.S. Pat. No. 406,328 is due to the fact that, in natural running, the leg absorbs the impact momentum by bending the knee to lower the runner's center of mass approximately 3 inches. The leg then lifts the center of mass by not only extending the knee 3 inches, but also the ankle, perhaps another 5 inches. That is, there is an asymmetry in the travel of the downward and upward action of the leg during running, referred to hereafter as vertical asymmetry. For a brace to function satisfactorily, it must mimic this vertical asymmetry. None of the Yagn inventions provide for this requirement.
Second, the telescopic design of U.S. Pat. No. 406,328 positions the bottom of the brace behind the foot, which means that its thrust will necessarily occur too soon to aid in running and with a smaller amount of travel than if the brace were positioned at the side of the foot. That is, the vertical asymmetry afforded by the Yagn design would be the opposite of that required to aid in natural running, and, hence, Yagn's invention could not work.
Third, the telescopic design of U.S. Pat. No. 406,328 does not allow the foot to be lifted a sufficient distance, during recovery, to allow for natural running. Its foot lift is limited to less than a third of the brace length. In natural running, the ability to lift the foot high improves performance. By shortening the leg system, its moment of inertia about the hip joint is decreased. This reduces the muscle energy required to accelerate the foot forward during recovery. This consideration becomes more important as running speed increases. Finally, the Yagn patents do not provide for a rapid means to stop.
My invention provides for these essential capabilities, i.e., vertical asymmetry, adequate foot lift, and rapid braking, which are not covered in Yagn's inventions or in any other prior art.